1. Field of the Invention
The present invention relates to a conductive paste for a multilayer electronic part, and particularly, to a conductive paste which is directly screen-printed on a ceramic green sheet. More particularly, the present invention relates to a conductive paste of high metal content for a multilayer electronic part which is suitable for efficiently forming a conductor circuit, an electrode, a via conductor, and the like, of a multilayer electronic part, such as a multilayered substrate, a multilayer inductor, a multilayer capacitor, and a multilayer actuator. In particular, the present invention relates to a conductive paste for a multilayer electronic part, which paste enables a continuous printing of a high-definition conductor pattern with a high aspect ratio on a low-temperature fired multilayered substrate.
2. Related Art
In recent years, an applied electronic equipment has rapidly been developed, and the requirements of the miniaturization of electronic parts and the change to high-density mounting have increased. Moreover, as for a substrate, on which electronic component is mounted, a multilayer substrate, in which elements such as an inductor and a capacitor are built, has been being developed. Furthermore, a low-temperature fired substrate (low temperature co-fired ceramics (LTCC) substrate) using a ceramic material or a glass-ceramic material, which LTCC substrate can be fired at a low temperature of 1,000° C. or less, is known for performing a co-firing with a conductor material and/or a resistive material, and the LTCC substrate is used as the substrate of a high-frequency superimposing module, an antenna switch module, a band-pass filter, a balun, a coupler, a deplexer, and the like.
The multilayer substrate is manufactured, for example, as follows. An organic binder, a plasticizer, a solvent, a dispersing agent if necessary, and the like, are suitably mixed with a substrate material such as glass powder and ceramic powder, the mixed powder is formed into a sheet to obtain a ceramic green sheet, and if necessary, the ceramic green sheet is drilled to form a via hole. A conductive paste is printed on the green sheet by a screen printing method to form a predetermined conductor pattern, and the conductive paste is filled in the via hole. A plurality of sheets on which the conductive paste pattern has been formed in such a way is laminated on one another and is cut. Subsequently, the cut laminated body is fired, and if necessary, a surface conductor is further formed.
Moreover, a multilayer electric part, such as a multilayer inductor, is manufactured as follows. A through-hole is formed in a ceramic green sheet including a magnetic material powder or a non-magnetic material powder. A coil conductor is formed as an internal electrode by screen-printing a conductive paste in a predetermined pattern. A coil is formed by laminating a predetermined number of the green sheets on which the electrode is formed. The laminated green sheets are cut. The cut green sheets are fired. Then, a terminal electrode is formed.
In both the cases, as for the conductive pastes, the followings are used; at least conductive powder, a vehicle including a resin and an organic solvent, and various additives at request are mixed to be a paste, a paint, or an ink.
As the conductor materials of the conductive pastes in these multilayer parts, metals such as gold, silver, copper, palladium, platinum, nickel, and tungsten have conventionally been used. For the high integration, the miniaturization, and the weight saving of an LTCC substrate, it is important to suppress transmission loss to be low. Accordingly, silver, copper, and alloys containing silver and/or copper, which have low resistance values, are mainly used for an LTCC substrate and a multilayer inductor. Moreover, in order to lower a conductor resistance, it is necessary to form a conductor film with a dence and thick configuration. Therefore, a paste having a large metal powder content, especially the one having a metal powder content in the paste of from about 70 weight % to about 95 weight %, is generally used. The situation is also true in the case of using the conductor film to a via hole conductor. That is, in order to heighten the filling property to a via and to minimize the difference of firing shrinkage between that of the substrate and that of the conductor, which difference causes an interlayer separation and a crack, it is necessary to suppress the firing shrinkage of the conductor. Accordingly, the amount of the resin and the solvent to be burnt off during firing is decreased, and the solid content of the conductive paste is heightened as much as possible.
On the other hand, the screen printing method, which is generally used for forming a conductor pattern on a ceramic green sheet, is an excellent printing technique having high productivity, but it is difficult for the screen printing method correspond to the requirement of the enhancement of accuracy accompanying the densification of the latest electronic equipment. That is, for example, it is required to form a fine pattern in which line width/space is 100 μm/100 μm or less, and it is also required to have a good filling property into the via at the same time. In particular, it is difficult to form a pattern in which the line widths are narrow and the aspect ratio (film thickness/width) thereof is large with high accuracy and high productivity while necessary printed film thickness is secured when the LTCC, and the multilayer inductor, or the like, are manufactured.
Conventionally, mass productive printing on green sheets has been performed by the optimization of a screen printer and a paste, and the property of the screen printing of the paste has been enhanced by controlling a viscosity, a thixotropy index (TI) value, an yield value, and the like, thereof. It is necessary to use a paste having high viscosity, a high TI value, and a high yield value for forming a line pattern having a large aspect ratio by screen printing. However, although a line pattern having a large aspect ratio can be formed by adjusting such factors suitably, it is difficult to control the occurrence of the bleeding of a paste, a low spot, a chip, a shear droop, and the like, of a pattern. And, there is the following problem, that is, when printing is repeated a number of times, these disadvantages arise to deteriorate the accuracy of printing, and to make it impossible to perform continuous printing.
The following matters can be considered to be the causes of the disadvantages such as the bleeding, the low spot, the chip, and the shear droop.
a) When the screen-transmission quantity of a paste is too much, a part of the paste enters the back of a screen printing plate to cause the bleeding.
b) When the screen transmission of a paste is bad, the clogging of the paste to a mesh arises, or the paste adheres to the side wall of a pattern hole of the screen. Consequently, a low spot or a chip arises in the pattern.
c) The paste which is screen-printed on a substrate cannot keep the shape thereof, and the line width thereof is widened. Consequently, a shear droop arises.
Because such disadvantages more remarkably arise as the number of repeating times of printing increases, the disadvantages cause a serious problem in a case where screen printing is continuously performed many times in a mass production line of a multilayered substrate or a multilayer component, and, for example, it is extremely difficult to perform highly accurate screen-printing with repetition of more than 1,000 times. Accordingly, it has conventionally been difficult by the related art to mass-produce, for example, a multilayered substrate or a multilayer component having a conductor pattern with a width of from 25 μm to 60 μm and a high aspect ratio.
Accordingly, for example, it is proposed to form a fine conductor pattern using a photolithographic method (photochemical engraving technique) with a photosensitive electroconductive paste instead of the screen printing method. The photosensitive electroconductive paste contains a conductive metal powder, such as the powder of silver, gold, and copper, a photosensitive resin, a photo initiator, a solvent, and the like. The paste is applied to the whole surface of the substrate. After the paste has dried, an ultraviolet ray is radiated using a photomask to cure the exposed area. Next, the parts of the paste that are not cured in the unexposed areas are removed using a developing solution to form a pattern. However, when a pattern is intended to be formed on a ceramic green sheet using such a photosensitive paste, the organic solvent in the paste permeates the green sheet, and thereby the removal of the paste in the unexposed area becomes very difficult at the time of development, to make it difficult to obtain a pattern with high-definition. Moreover, because the metal powder in the paste disturbs the penetration of the ultraviolet ray into the inner part of the coated film, it is difficult to cure the coated film to the inner part thereof sufficiently, especially when the coated film is thick. Consequently, it is difficult to form a conductor of a thick film thickness.
Japanese Patent Application Laid-Open Publication No. 2003-124052 describes that the problems such as bleeding, clogging, cobwebbing, chipping of a paste by a squeegee, and the like, can be settled by setting the following conditions in a conductive paste containing a conductive metal powder and an organic vehicle which conductive paste is suitable for being printed on a ceramic green sheet to form an internal electrode of a multilayer capacitor, and the like. The conditions are, the viscosity at the shear rate of 500 s−1 at the temperature of 25° C. is 1.0-10.0 Pa·s, and the viscosity at the shear rate of 10 s−1 is 5.0-20.0 Pa·s. Furthermore, the ratio of a shear storage modulus to a shear loss modulus (tan δ) at a frequency of 1 Hz is within a range of from 2.0 to 8.0. The description is based on the concept that the disadvantages, such as the bleeding and the cobwebbing and the like, at the time of screen printing arise because of the quickness of the structure recovery when the paste is released from the viscosity under a high shear rate at the time of printing and changes to the viscosity under a low shear rate. Further, the concept is that the quickness is influenced by the ratio of a viscous component (shear loss modulus in a dynamic viscoelastic measurement) to an elastic component (shear storage modulus in the dynamic viscoelastic measurement) that constitute the paste. The method disclosed in the description attempts to solve the problems by adjusting the ratio, tan δ, of the shear storage modulus to the shear loss modulus into a specific range at a specific frequency of 1 Hz, based on the concepts.
However, by research of inventors of the present invention, such a method is effective for a paste in which the contained amount of the metal powder is not so large, such as a paste for the internal electrode of a multilayer capacitor, which is required to form an extremely thin conductor film, as the preferable contained amount of the metal powder disclosed in the Japanese Patent Application Laid-Open Publication No. 2003-124052 is within a range of from 40 weight % to 60 weight %. But, the method cannot solve the problem of the continuous printing property in a paste in which the content of the metal powder is high, for example, 70 weight % or more, and which is to be used for forming a conductor film having a comparatively thick film thickness and a low resistance value as described above.